The present invention relates to stable calcium double salts which contain, in one molecule, one calcium atom (cation) and one radical each of two different organic acids (anion) in the same stoichiometric ratio, preferably acids such as formic acid, acetic acid, propionic acid, sorbic acid and benzoic acid. In addition, the invention relates to formulations that comprise the double salts and a stabilizer and/or a formulating aid.
The invention further relates to a process for preparing the double salts, and to the use of the salts in foods, cosmetics and drugs, food-contact articles, feeds, for example in silage, brewer""s spent grains, pomace, food wastes, brewer""s yeast, distillation residues and other wastes from the food industry, or as preservative in industrial products.
Antimicrobial substances which are less physiologically harmful are increasingly replacing substances which are hazardous to health or the environment, for example antibiotics, formaldehyde-releasing substances, halogenated substances, boric acid derivatives, and many others, in foods, feeds, pet food, cosmetics and drugs, food-contact articles, silage, brewer""s spent grains, pomace, food waste, brewer""s yeast, distillation residues and other wastes from the food industry or in leather treatment. Substances which are less physiologically hazardous include the classic preservatives, in particular sorbic acid, propionic acid or benzoic acid. These organic acids are used in particular in the food industry Antimicrobial Food Additives Characteristics, Uses, Effects by Erich Lxc3xcck and Martin Jager, (2nd revised and enlarged edition, Springer)
However, increasingly, mixtures of organic acids are being used. A frequent disadvantage is the liquid state, their volatility or poor solubility even of some of their sodium salts, potassium salts, calcium salts or magnesium salts. Poor miscibility or incompatibility of these acids is frequently observed. Thus, for example, mixtures of formic acid and benzoic acid exhibit a rapid brown discoloration, which is possibly due to oxidation reactions.
Preservative acids generally act in their undissociated form. This dependence on the pH gives the necessity of using high concentrations of some preservatives in less acidic foods which, for example in the case of the use of propionic acid in bread, can very rapidly become noticeable by disadvantageous sensory properties.
Differing product properties of the products to be preserved, in particular differing consistency and differing pHs lead to the fact that there is no generally optimum preservative, but that product-specific solutions in preservative are required (M. Jager and E. Lxc3xcck: see above). However, in many cases, such solutions mean that, in addition to one preservative, other substances must be added, and there is thus the necessity, for example, of adding various preservatives one after the other and keeping stores of them.
DE-A 197 39 319 describes acid-impregnated carboxylic acid salts, inter alia, or calcium formate impregnated with propionic acid. The invention only relates to the mixing of salts and acids, the acid physically adsorbing to the salt, with no chemical reaction occurring.
It was an object of the present invention to prepare a defined product that combines the advantages of two preservative acids. In particular, a storage-stable product was to be prepared that can withstand without loss relatively long transport and storage times, or poor storage conditions. In addition, it was an object to prepare products that can readily be employed in the food or related industries.
This object is achieved by calcium double salts containing two different acid units per double salt molecule. It has been found that, in addition to the calcium salts of organic acids which have long been known, a whole series of calcium double salts containing two different acids in the molecule can be prepared, in which various combinations of acid anions are possible.
The invention therefore relates to calcium double salts of the formula I:
Ca(R1)(R2)xe2x80x83xe2x80x83(I) 
where R1 and R2 are different and are each OOCxe2x80x94R3, where R3=saturated or monounsaturated or polyunsaturated C1-C5-alkyl, C1-C5-hydroxyalkyl or phenyl. Preference is also given to compounds in which customary preservative acids are used, that is to say compounds of the formula (I), where R3 is methyl, ethyl, hydroxyethyl (=lactate anion), propyl, 1,3-hexadienyl (=sorbate anion) or phenyl. Very particularly preferably, R1 is propionate and R2 is sorbate.